1. Technical Field
This invention relates generally to maximizing the efficiency of radio frequency power amplification in a wireless communication device transmitter, and, more particularly, to a high efficiency multiple power level amplifier.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld telephone-like communication handset. While the different modulation and transmission schemes each have advantages and disadvantages, one common factor is the need for highly efficient power amplification. As these communication devices become smaller and smaller, the functionality provided by these devices continues to increase. One major concern when developing these handheld communication devices is power consumption. As the devices become smaller and smaller, the amount of power consumed and dissipated becomes more and more critical. High efficiency power amplification decreases the amount of power consumed, thereby maximizing battery life of the device.
Another major concern in these wireless devices is the size of the circuitry. In order to minimize the hardware required it is desirable to integrate as much functionality as possible into fewer and fewer circuit modules. This enables the handheld device to be smaller and consume less power.
Many wireless power amplifier applications require high efficiency over a broad range of operating power levels. This is inherently difficult to achieve without circuitry and logic in addition to the power amplifier. Typically, additional circuitry residing on a control die must be used in addition to the power amplifier circuit.
FIG. 1 is a simplified block of a typical transceiver 50. Transceiver 50 includes a bias circuit 100, a power amplifier 120 and a voltage regulator 140. Bias circuit 100 maintains a constant current IB to power amplifier 120 based upon a reference voltage Vref provided to the bias circuit 100 by the voltage regulator 140.
Bias control systems to control the level of voltage bias applied to a power amplifier, and thus the level of power consumed by the power amplifier during operation, are often used in conjunction with wireless communications devices incorporating power amplifiers. One example of such a bias control system is illustrated in FIG. 2. In this example, an emitter follower bias circuit 100 is illustrated. The emitter follower bias circuit 100 provides a base current IB required by a radio frequency (RF) power amplifier 120, and more particularly, RF transistor 32 for direct current (DC) bias and RF power conditions. Both emitter follower bias circuit 100 and power amplifier 120 are typically implemented using the same semiconductor technology, for example, gallium arsenide (GaAs) heterojunction bipolar transistor (HBT).
One of the primary disadvantages of this type of common bias control system when implemented using GaAs HBT technology is that due to the two base emitter voltage drops across buffer transistor 30 and RF transistor 32, respectively, Vref must be greater than +3.0V to maintain adequate operation over the operating temperature range as the base to emitter voltage drop VBE of each of these transistors is approximately +1.3 volts each. However, in many communications devices, such as mobile cellular or PCS telephones, batteries are used to provide a supply voltage to the communications device. These batteries are typically configured to provide a minimum operating voltage of +2.8 VDC. Communications devices are often configured to shut off when the available supply voltage falls below +2.8 volts DC (VDC). Once the available battery voltage drops below +3.0 VDC, it is necessary for steps to be taken to boost the sub +3.0 VDC operating voltage supplied by the battery up so that the voltage supplied to the communications device as VDC is the required +3.0 volts. This requires additional circuitry to boost the sub +3.0 VDC voltage and provide a regulated voltage to the communications device that is greater than the minimum battery voltage.
Further, as an external voltage is typically required to provide a reference voltage VREF to the bias circuit 100, an external input 49 is provided to connect an external voltage supply to the bias circuit 100. In RF communications devices, electrostatic discharge (ESD) can damage the circuitry of the communications device. ESD may be propagated through the circuitry of the communications device via connections between circuitry/components. The presence of an external input 49 reduces the reliability of the bias circuit 100, as well as the communications device 150 in general, as it increases the risk of ESD being picked up and propagated through the bias circuit 100, thereby potentially damaging the bias circuit 100 and/or power amp 120. GaAs HBT technology typically provides resistance to ESD of up to xc2x11 kilovolt (1 KV). ESD exceeding xc2x11 KV is common and jeopardizes circuitry of the communications device.
Additionally, in the communications device 150, the base current (IB)RF provided to the RF transistor 32 of power amplifier 120 is prone to shift as the power required by RF transistor 32 increases/decreases. Thus, in order to compensate for such shifting in bias current, it is common to provide a higher bias voltage to the base of the RF transistor 32. This leads to lower efficiency, greater consumption of power and the need for a higher supply voltage.
The bias circuit 100 is typically configured to provide a quiescent current (IB) to the RF transistor 32 that allows for maximum gain and linearity at the maximum RF output power level. However, at low power levels this fixed quiescent current is higher than necessary for proper operation at the lower power levels. As a result the efficiency of the power amp 120 diminishes at lower RF output levels.
The voltage at node 34 is established by the base-to-emitter drop of the mirror transistor 26 and the buffer transistor 30. The voltage at node 34 establishes the reference current Iref which flows through the resistor Rref. As the base to emitter voltage drop of a transistor fluctuates as temperature fluctuates any changes in temperature impact the voltage at node 34. Thus, as the temperature changes and the base to emitter voltages across mirror transistor 26 and buffer transistor 30 change, the voltage at node 34 changes. This results in the current Iref also changing. As Iref varies so will the output current IC at RF transistor 32. Unfortunately, as the current IC decreases so does the linearity of RD transistor 32.
Therefore, there is a need in the industry for a wireless power amplification circuit that achieves highly efficient power amplification over a broad range of output power levels and that is economical to produce in high volume.
The present invention provides a system for biasing a power amplifier in a communications device. Briefly described, in architecture, the system can be implemented as follows. A band gap voltage generator for generating a bandgap voltage is provided to a voltage-to-current converter. The voltage-to-current converter generates a reference current in accordance with the bandgap voltage. The reference current is provided to a programmable current mirror that multiplies the reference current to a predetermined level. A feedback amplifier is provided for outputting and maintaining a constant current to a reference device.
Related methods of operation and computer readable media are also provided. Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.